All publications herein are incorporated by reference to the same extent as if each individual publication or patent application was specifically and individually indicated to be incorporated by reference. The following description includes information that may be useful in understanding the present invention. It is not an admission that any of the information provided herein is prior art or relevant to the presently claimed invention, or that any publication specifically or implicitly referenced is prior art.
As the cost of fossil fuels increases and the scarcity of potable water becomes more acute, the need for alternative methods of freshwater production becomes greater each year. In fact, much of the developing world's population lives where potable water is extremely rare. Therefore, an easily manufactured, simply operated means for producing fresh water from brackish water would be a critical step in providing the basic necessities of life for literally millions of people. Solar distillation is perhaps the most basic method for generating potable water from seawater or brackish water, and simple solar distillation systems have been in use in one form or another for hundreds of years.
Several issued U.S. patents specifically relate to machines that use the sun's energy to evaporate seawater in a closed reactor. The steam vapor generated rises above the water surface and condenses on the inside cover of the reactor. The distillate collected is suitable as potable water. Examples of “solar stills” can be found in U.S. Pat. No. 6,767,433 (Foster et al. 2004), U.S. Pat. No. 6,001,222 (Klein 1999), U.S. Pat. No. 4,235,678 (McKeen 1980), and U.S. Pat. No. 4,135,985 (LaRocca 1979). These devices, ranging in size from personal fresh water generators to building-sized units, generally involve an insulated box, a pan or pool of water, and a transparent top that allows light to enter but traps heat and vapor.
Numerous improvements have been developed to increase the efficiency of such devices, including lowering the air pressure above the water surface (thereby lowering the amount of heat required for generating vapor), using forced or induced ventilation to increase evaporation rates, using cover “shakers” to help condensed water migrate down the cover to the collection trough, and separating the evaporation chamber from the condensation chamber to increase thermal efficiency. Floating units have been patented that utilize the ocean or lake as a heat sink for the condenser, see, for example, U.S. Pat. No. 6,656,326 (Nagler 2003). Because these devices rely on direct solar heating, their effectiveness is limited by the size of the device.
Another broad class of inventions utilizes inclined or horizontal tubes for the evaporation chamber. The tubes are located at the focal point or focal line of a reflector, or in some cases, they are located at the focal point of a Fresnel lens. Examples of these devices can be found in U.S. Pat. No. 5,505,917 (Collier 1996), U.S. Pat. No. 5,191,876 (Atchley 1993), and U.S. Pat. No. 4,312,709 (Stark et al. 1982). U.S. Pat. No. 2,141,330 (Abbot 1938) shows an early solar distilling apparatus that uses a clockwork mechanism to track the sun's path through the sky. In this and similar devices, the fluid is heated as it travels through the tubing, similar to a conventional boiler. The steam generated can be used for work, heat transfer, heat storage, or, if conveyed to a condenser, the steam can be returned to liquid distillate. Unfortunately, these devices are susceptible to mineral (scale) buildup in the evaporation tubes. This can be partially overcome by the use of a closed heating circuit with an engineered working fluid, but this solution adds another level of complexity and additional heat transfer loss between loops.
Vertical arrangements for solar evaporators are less numerous. One arrangement is shown as part of a larger system in U.S. Pat. No. 4,373,996 (Maruko 1983). Vertical distillation towers for alcohol have been patented, e.g. U.S. Pat. No. 4,377,441 (Kimmell 1983), U.S. Pat. No. 4,455,374 (Schwarz 1984), and U.S. Pat. No. 102,633 (Wheeler et al. 1870). The '441 patent shows a solar distilling apparatus with a vertical reactor and automated fill and relief valves. The design shows a parabolic reflector arranged to focus sunlight onto the bulbous “boiler” section of the reactor. This design shows one advantage of a vertical reactor arrangement, that of compact size, but it also shows the difficulty in supporting the reactor column at sizes larger than desktop units. In addition, the device uses complex valves to maintain suitable fluid levels and temperatures within the boiler and distilling tower.
Providing a cost-effective and workable method to separate the raw feed liquid from the concentrated liquid left behind after the vapor has been driven off is a problem common to all previous designs. Batch mode operation is one method, but that solution eliminates continuous production and potentially wastes part of the daily sun cycle.
Therefore, there is a need in the art for a simple, cost-effective and easily manufactured device that can be employed to desalinate, purify, or distill water or other liquids.